Influence of crop background on aphids and other phytophagous insects on Brussels sprouts

More Brevicoryne brassicae and other alate aphids were caught in yellow water-traps in a weed-free crop of Brussels sprouts than in a crop with a weedy background. More B. brassicae colonized Brussels sprout plants in bare soil than in weeds; very few alatae were attracted to cruciferous weeds in the crop. Results in 1 yr suggest that initially larger populations on the weed-free sprouts became smaller than populations on the weedy sprouts because the larger aphid population attracted more natural enemies. Aleyrodes brassicae and certain Lepidoptera were also more abundant on sprout plants in bare soil than on sprouts surrounded by weeds; more adult A. brassicae were caught in water traps over the bare soil. More A. brassicae were present on sprout plants surrounded by a green than by a brown cloth background but the differences were not significant (P < 0–05). Numbers of B. brassicae on sprout plants with green and brown backgrounds varied greatly and did not differ significantly. In field cages, B. brassicae alatae were more attracted to potted sprout plants surrounded by bare soil than to ones surrounded by rings of living or cut grass or by artificial green rings. This effect was greater with small than with large sprout plants surrounded by grass rings. The maintenance of a limited weed cover is considered potentially useful in integrated control of some brassica pests.     

The relationship between aphid infestations and oviposition by aphidophagous Syrphidae (Diptera)

Oviposition by aphidophagous Syrphidae varies with the size of aphid infestations and different syrphid species have different optimum aphid population sizes for oviposition. In one experiment using potted brussels sprout plants infested with Brevicoryne brassicae L., Platycheirus manicatus (Meig.) preferred about 100 aphids per plant, P. scutatus (Meig.) about 1000 aphids per plant and Syrphus ribesii (L.) about 2000 aphids per plant. These preferences were less clear in certain Platycheirus species (e.g. P. peltatus (Meig.)), than in Syrphus species (e.g. S. luniger Meig.). Once a plant has been selected for oviposition there may still be selection of suitable colonies on that plant. For a given aphid population, S. luniger preferred a small number of large aggregates to a larger number of smaller ones, whereas S. balteatus (Deg.) preferred the opposite. The tendency of the different species to select aphid populations of different sizes and distributions is likely to decrease interspecific competition.     

Resistance to attack by Brevicoryne brassicae among plants of Brussels sprouts

Because they remained almost uncolonized by the cabbage aphid (Brevicoryne brassicae (L.)) throughout the growing season, plants of Brussels sprouts were singled out in each of 4 years, from plots heavily infested with the aphid, as possibly being resistant to attack. Clones of these plants were established from cuttings and tested in a controlled environment by inoculation with B. brassicae and later, in the field, by natural infestation. The tests confirmed that some of the plants were resistant to the aphid, and the most resistant of those from the first year of the work proved at least as resistant as any subsequently found. The resistance was expressed as antibiosis, but in the field host non-preference was also shown by incoming winged aphids. The possibility that biotypes of B. brassicae might exist, to which the resistant sprout clones were not necessarily resistant, was investigated using B. brassicae collected from sprouts from each of several areas in England. Eight sprout clones, seven of which were known to be resistant, and the other susceptible, to B. brassicae from Wellesbourne were tested with these other B. brassicae. The results showed that biotypes of the aphid, with differing abilities to colonize respective sprout clones, existed in each area, and of the seven sprout clones resistant to the Wellesbourne aphid, only one appeared never to be fully susceptible to one or more of the other biotypes of B. brassicae.     

Maternal host plant effects on aphid performance: contrasts between a generalist and a specialist species on Brussels sprout cultivars

• 1 The performance of the second generation (G₂) of alates and apterae of a generalist, Myzus persicae, and a specialist, Brevicoryne brassicae, aphid species reared on Chinese cabbage or cabbage was evaluated on five cultivars of Brussels sprout.
• 2 Aphid performance was influenced both by the type of host on which the parent aphid had been reared and by the host on which it was feeding when reproducing.
• 3 The fecundity of the G₂ of alates of both aphid species reared on Chinese cabbage differed significantly between all the cultivars of Brussels sprout and, on average, was 25% higher than those reared on cabbage. These differences were also apparent for the intrinsic rate of increase of B. brassicae but not for M. persicae.
• 4 There was a trend for the G₂ of alates from Chinese cabbage to have greater fecundity compared with aphids from cabbage. These differences were significant for the fecundity of the G₂ of alates of both aphid species on Brussels sprout cultivars Fillbasket (30% higher), Red Delicious (35% higher) and Winter Harvest (25% higher) than those reared on cabbage.
• 5 The intrinsic rate of increase for the G₂ of alates of B. brassicae from Chinese cabbage was significantly different on all Brussels sprout cultivars tested. The intrinsic rate of increase differed significantly between aphids reared on either Chinese cabbage or cabbage on cultivars Oliver and Darkmar‐21 (M. persicae) and Red Delicious and Winter Harvest (B. brassicae). The cv. Oliver appeared to be the most consistently good host; Red Delicious was the poorest host overall.

     

The use of oviposition‐induced plant cues by Trichogramma egg parasitoids

1. Female parasitoids have evolved various foraging strategies in order to find suitable hosts. Egg parasitoids have been shown to exploit plant cues induced by the deposition of host eggs. 2. The tiny wasp Trichogramma brassicae uses oviposition‐induced cues from Brussels sprouts to locate eggs of the cabbage white butterflies Pieris brassicae and Pieris rapae that differ in their egg‐laying behaviour. These plant cues are elicited by male‐derived anti‐aphrodisiac pheromones in the accessory reproductive gland (ARG) secretions of mated female butterflies. However, the closely related generalist species Trichogramma evanescens does not respond to Brussels sprout cues induced by the deposition of P. brassicae egg clutches. 3. Here we showed in two‐choice bioassays that T. evanescens wasps respond to Brussels sprout cues induced by (i) the deposition of single eggs by P. rapae, and (ii) the application of ARG secretions from either mated P. rapae females, or from virgin female butterflies in combination with P. rapae's anti‐aphrodisiac compound indole. The wasps only associatively learned to respond to Brussels sprout cues after applying indole alone by linking those cues with the presence of P. rapae eggs. 4. Our results indicate that Trichogramma wasps more commonly exploit oviposition‐induced plant cues to locate their host eggs. Generalist wasps show less specificity in their response than specialists and employ associative learning.     

Seasonal changes in aphid populations on Brussels sprouts

Populations of Brevicoryne brassicae (L.) and Myzus persicae (Sulz.) were assessed weekly on Brussels sprouts at Wellesbourne, from July onwards in each of 3 years. A peak of B. brassicae numbers occurred every year about the end of July, and in 1968 and 1970 this was larger than the September peak. Some alates were produced in the generation born to the June immigrants and the dispersal of those produced at the end of July consistently brought a decline in B. brassicae populations in early August. In 1968 and 1970, however, outbreaks of entomogenous fungi which followed rain, added considerably to this decline. Records of predators and parasites indicated that these natural enemies had only a minor effect on aphid numbers. M. persicae colonized sprouts slightly earlier than B. brassicae; in 1969 and 1970 its numbers in July were low and some persisted after the first week of August in 1968 only. In each year the first eggs of B. brassicae were laid at the beginning of October, but viviparae were still numerous for a further 7 to 10 weeks.     

Can imitation companion planting interfere with host selection by Brassica pest insects?

• 1 Companion planting with nonhosts may offer a non‐insecticidal means of controlling pests, although the results of studies can be variable and species‐dependent.
• 2 The effect of companion planting on two pests of Brassica crops, Plutella xylostella (L.) and Brevicoryne brassicae (L.), was examined using Brussels sprout as the host plant and imitation cereal plants made from green plastic as the nonhost. For P. xylostella, the effect of nonhost density was also investigated.
• 3 Oviposition (P. xylostella) and abundance (B. brassicae) were lower on Brussels sprout plants presented on a background of high‐density imitation cereal plants (reductions of 59% and 85%, respectively).
• 4 The results are discussed in the context of host location by pest insects and the selection of nonhost companion plants for pest management.
• 5 It is concluded that nonhost plants interfere with pest host selection through disruption to visual host location processes.

     

Experiments on integration of chemical and biological control of aphids on brussels sprouts

Two field experiments were made to test whether natural enemies would take over control of brussels sprouts aphids at the time when protection from a selectively acting, soil-applied systemic insecticide, menazon, began to fail. The natural enemies, notably Syrphidae, proved ineffective against Brevicoryne brassicae L. despite advantages given by the insecticide and by close planting, which greatly increased the ratio of numbers of syrphid eggs and larvae to aphids. Thus, development of the aphid population was little altered when infested plants were kept free of most natural enemies by hand removal. I lb/acre (1–12 kg/ha) of menazon applied as spot treatments to the soil at planting-out time decreased the number of overwintering parasite mummies by 70 % but such ‘mortality’ was compensated for by decreased mortality from hyperparasites and other causes, so the numbers of adults of the primary parasite Diaretiella rapae (McIntosh) which emerged in the spring were similar to those from untreated plots. Soil cultivation in winter drastically decreased the numbers of emerging adult primary parasites, hyperparasites and syrphids. The menazon treatment designed for integrated control (1 lb/acre) seemed too unpredictable in action, e.g. less effective in dry than in damp conditions, to provide the hoped-for chemical control needed until natural enemies became abundant. Menazon at 4 lb/acre (4·48 kg/ha) protected the crop throughout the growing season and bioassays showed that menazon or its toxic derivatives continued to occur in leaves during cool periods in winter and also in the following April-May nearly 1 year after application. The same amount of menazon per unit area of crop was less effective on 1 ft 6 in (45·6 cm) spaced than on 3 ft (91·2 cm) spaced brussels sprouts plants.     

Glasshouse evaluation of some systemic fungicides for control of clubroot of brassicae

Of nine systemic fungicides screened as soil mixes against clubroot, only the precursors of methyl benzimidazol-2-ylcarbamate (MBC) or ethyl benzimidazol-2-ylcarbamate (EBC) gave promising results. Benomyl was the most effective, usually giving control at 250 mg/kg dry soil. Most fungicides were less effective against an isolate of Plasmodiophora brassicae from Brussels sprouts than against one from rape. Disease control was slightly better on cabbage than on a highly susceptible rape variety.     

Hoverflies (Diptera,Syrphidae) in Agrolandscapes of St. Petersburg and Leningrad Province

A total of 57 species from 32 genera of hoverflies (Diptera, Syrphidae) were found in the agricultural landscapes of St. Petersburg and Leningrad Province. One species, Cheilosia vulpina (Mg.), was recorded in the region for the first time. The largest numbers of species occurred at forest and field edges covered with wild grasses and shrubs, while 21 species of hoverflies were recorded in crops. The species most commonly found in crops were representatives of the genera Melanostoma, Sphaerophoria, and Platycheirus whose larvae feed on aphids.     

Distribution of elemental interactions in Brussels sprouts plants,under the Treated Municipal Wastewater

Abstract

The distribution of the macro, micronutrients and heavy metal interactions in the various plant parts (roots, leaf, and sprout) of Brassica oleracea var. gemmifera (Brussels sprouts), was investigated in a greenhouse experiment of randomized block design, in Agrinion, Greece. The statistical design included two variables: (i) Treated Municipal Wastewater (TMWW), and (ii) fresh irrigation well water (control). The analytical data of plant and soil samples collected were processed statistically by means of regression analysis, ANOVA and t-test, using an SPSS package. The ultimate goal of the experiment was to establish a scientific basis for the safe re-use of TMWW in the irrigation of Brussels sprouts, and possibly of all vegetables, with least accumulation of heavy metals in the sprouts.     

Effects of intercropping with Spergula arvensis on pests of Brussels sprouts

Intercropping as a method to reduce crop losses is an old technique, especially in the tropics. During 1977 and 1978 intercropping experiments of Brussels sprouts with Spergula arvensis showed a significant pest reducing effect. Mamestra brassicae and Evergestis forficalis populations showed a strong reduction, whereas Pieris rapae did not seem to respond to intercropping. Late cabbage root fly infestation of the sprout buttons was reduced drastically. Brevicoryne brassicae incidence was decreased in dense intercropping plots only. Injury due to caterpillar feeding showed a grading response to intercropping density, as did the total caterpillar populations. Log dose-effect regression lines allow a comparison of various intercropping densities with other control methods on insect pest population reducing potential.

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Zusammenfassung Durch Untersaaten können Ertragsverluste an Kulturpflanzen reduziert werden. Das ist ein altes, bewährtes Verfahren, besonders in den Tropen. Die Einsaat von Spergula arvensis in Rosenkohlbeständen führte in 2-jährigen Untersuchungen zur signifikanten Verringerung des Schädlingsbefalls. Während die Populationen von Mamestra brassicae und Evergestis forficalis stark zurückgingen, blieb die von Pieris rapae unbeeinflusst. Eine Abnahme von Brevicoryne brassicae konnte nur in Parzellen mit hohen Spergula-Dichten festgestellt werden. Die Abundanz der Raupen sowie die Beschädigungsintensität durch diese, zeigten eine Abhängigkeit von der Untersaatdichte. Die Regressionsgerade Untersaatdichte zur Effektivität erlaubt einen Vergleich verschiedener Spergula-Dichten mit anderen Methoden, die das Schädlingspotential reduzieren.

     

All roads lead to weediness: Patterns of genomic divergence reveal extensive recurrent weedy rice origins from South Asian Oryza

Weedy rice (Oryza spp.), a weedy relative of cultivated rice (O. sativa), infests and persists in cultivated rice fields worldwide. Many weedy rice populations have evolved similar adaptive traits, considered part of the ‘agricultural weed syndrome’, making this an ideal model to study the genetic basis of parallel evolution. Understanding parallel evolution hinges on accurate knowledge of the genetic background and origins of existing weedy rice groups. Using population structure analyses of South Asian and US weedy rice, we show that weeds in South Asia have highly heterogeneous genetic backgrounds, with ancestry contributions both from cultivated varieties (aus and indica) and wild rice. Moreover, the two main groups of weedy rice in the USA, which are also related to aus and indica cultivars, constitute a separate origin from that of Asian weeds. Weedy rice populations in South Asia largely converge on presence of red pericarps and awns and on ease of shattering. Genomewide divergence scans between weed groups from the USA and South Asia, and their crop relatives are enriched for loci involved in metabolic processes. Some candidate genes related to iconic weedy traits and competitiveness are highly divergent between some weed‐crop pairs, but are not shared among all weed‐crop comparisons. Our results show that weedy rice is an extreme example of recurrent evolution, and suggest that most populations are evolving their weedy traits through different genetic mechanisms.     

Spectral reflectance from a broccoli crop with vegetation or soil as background: influence on immigration byBrevicoryne brassicae andMyzus persicae

Light reflectance in five wavebands of the spectrum was measured from broccoli (Brassicae oleracea var.botrytis [L].) interplanted with leguminous cover crops (cover crop background) or broccoli grown as monoculture (bare soil background), and fertilized with compost or synthetic fertilizer. AlateBrevicoryne brassicae (L.) andMyzus persicae (Sulzer) (Homoptera: Aphididae) were monitored in yellow pan water traps and on broccoli leaves. Reflectance intensity was higher with a background of bare soil at all wavebands except blue (400–455 nm) in the early-season. Intensity decreased as broccoli canopy cover increased at all wavebands except blue and green (515–550 nm), declining-most dramatically in the yellow (550–590 nm). Highest late-season intensities were in plots with bare soil background and fertilized with compost (those stressed for nitrogen). Few differences in spectral composition, expressed for each waveband as a percentage of total intensity, were recorded. Numbers of alatae were lowest in cover crop background plots in the early season, reached equivalency with bare soil background by mid-season, and showed highest positive correlations with intensity in the yellow (550–590 nm). Results correspond to laboratory findings that aphids are attracted to higher intensity light, especially in the yellow waveband, and support a phototactic explanation for aphid orientation in the field.     

Action of mephosfolan on anthocorid predators of Phorodon humuli

In glasshouse and field studies, soil drenches of the systemic insecticide mephosfolan were applied to hop plants to control the aphid Phorodon humuli, and to study the toxicity to anthocorid predators. The mortality of Anthocoris nemorum and A. nemoralis eggs in the leaf tissues increased with the dosage of mephosfolan while A. confusus eggs, which were laid in the petioles and veins, appeared to be unaffected at the dosages used, up to 2 g a.i./plant. Mortality of A. nemorum eggs, which were laid mainly in the leaf margins, was higher than in A. nemoralis eggs, which were laid mainly in the interveinal areas. For both species mortality was highest for the eggs laid in the leaf margins, and susceptibilities appeared similar. It is suggested that mephosfolan becomes unevenly distributed within the leaves, being most concentrated in the leaf margins and least concentrated in the veins and petioles. No species showed any preference when offered mephosfolan-treated and untreated leaves for oviposition, and the oviposition rates were unaffected by various mephosfolan dosages. Oviposition rates, larval mortality and duration of larval development were unaffected when specimens of each species were reared from eclosion to maturity on a diet of either mephosfolan-killed or heat-killed P. humuli.     

Falling aphids enhance impact of biological control by parasitoids on partially aphid-resistant plant varieties

Cage experiments, in which the population increase of Metopolophium dirhodum was measured either in the absence or presence of the parasitoid Aphidius rhopalosiphi, showed reductions of peak populations in the presence of the parasitoid of 30% on the susceptible wheat cultivar Armada, but of 57% on the partially resistant Rapier. On both cultivars, aphid population curves in the presence and absence of the parasitoid diverged before the first mummies were formed. The number of aphids leaving plants was measured, and was found to increase in the presence of the parasitoid, particularly with Rapier (almost double). Most aphids (about 75%) which left plants fell, and a smaller proportion regained plants when parasitoids were present than when they were absent. A field experiment with Brevicoryne brassicae and natural predation on Brussels sprouts also showed an increase in aphids leaving a less susceptible compared with a more susceptible cultivar.     

Feeding behavior of Brevicoryne brassicae in resistant and susceptible collard greens genotypes: interactions among morphological and chemical factors

The cabbage aphid, Brevicoryne brassicae (L.) (Hemiptera: Aphididae), is distributed throughout the tropical and subtropical areas of the world. The main crops attacked by B. brassicae are cabbage, collard greens, broccoli, Brussels sprouts, and cauliflower. To survive the attack of pest insects, plants have evolved various resistance mechanisms that may affect pest feeding behavior. The use of electronic monitoring through EPG (electrical penetration graph) can help characterize and distinguish the resistance mechanisms involved. This study evaluated the feeding behavior of B. brassicae in eight genotypes of collard greens, Brassica oleraceae L. var. acephala (Brassicaceae), exhibiting antixenosis and/or antibiosis resistance to this insect. Possible correlations were established between the glucosinolate levels, the hardness, and the epicuticular wax on the leaves vs. aphid feeding behavior. On the genotypes 22V, 5E, and 27VA, for which many ‘potential drop’ waves were performed, aphid development was slower, indicating antixenosis as resistance type. Aphids on the genotypes 22V and 24X required more time until accessing the phloem, also suggesting antixenosis as resistance category. Genotypes 22V and PE had hard leaves, which also points at antixenosis. Genotypes 20T and HS had higher total wax and wax mg⁻¹. Feeding parameters on ARI and 24X were similar to those observed on HS; antibiosis is likely to be the predominant resistance category of this germplasm. Because HS was considered as a susceptible standard genotype in this study, a higher gluconapin amount indicates that this compound does not influence cabbage aphid feeding behavior. The present study confirms that analysis of the physical and chemical aspects of collard greens genotypes by the EPG technique can provide a useful approach for the study of plant resistance to cabbage aphids.     

Analysing spatial patterns of spread of Lettuce necrotic yellows virus and lettuce big-vein disease in lettuce field plantings

Spatial patterns of spread of lettuce big‐vein disease (LBVD) and Lettuce necrotic yellows virus (LNYV) were examined in two plantings each consisting of two blocks of lettuce. LBVD came from planting land infested with viruliferous Olpidium brassicae resting spores, while LNYV was introduced by aphid vectors from external sources consisting of LNYV‐infected sowthistle (Sonchus oleraceus) weeds. Clustering of LBVD was obvious in an area where the soil was heavily infested with only sporadic occurrence elsewhere. There was a steep decline in LNYV incidence over distance from a concentrated external weed source, with clustering of LNYV‐infected plants at the crop edge closest to it. There was no evidence of secondary spread with LBVD or LNYV.     

Combined effects of patch size and plant nutritional quality on local densities of insect herbivores

Plant–insect interactions occur in spatially heterogeneous habitats. Understanding how such interactions shape density distributions of herbivores requires knowledge on how variation in plant traits (e.g. nutritional quality) affects herbivore abundance through, for example, affecting movement rates and aggregation behaviour. We studied the effects of plant patch size and herbivore-induced differences in plant nutritional quality on local densities of insect herbivores for two Brassica oleracea cultivars, i.e. white cabbage and Brussels sprouts. Early season herbivory as a treatment resulted in measurable differences in glucosinolate concentrations in both cultivars throughout the season. Herbivore induction and patch size both influenced community composition of herbivores in both cultivars, but the effects differed between species. Flea beetles (Phyllotreta spp.) were more abundant in large than in small patches, and this patch response was more pronounced on white cabbage than on Brussels sprouts. Herbivore-induction increased densities in all patches. Thrips tabaci was also more abundant in large patches and densities of this species were higher on Brussels sprouts than on white cabbage. Thrips densities were lower on induced than on control plants of both cultivars and this negative effect of induction tended to be more pronounced in large than in small patches. Densities of the cabbage moth (Mamestra brassicae) were lower on Brussels sprouts than on white cabbage and lower on herbivore-induced than on uninduced plants, with no effect of patch size. No clear effects of patch size and induction were found for aphids. This study shows that constitutive and herbivore-induced differences in plant traits interact with patch responses of insect herbivores.     

Population studies ofBrevicoryne brassicae (L.), its parasites and hyperparasites in England

Studies on populations of Brevicoryne brassicae (L.), its parasites and hyperparasites were carried out by actual counting in the sprouts field and by sticky and water traps. B. brassicae was found to be attacked by one primary parasite, Diaretiella rapae (McIntosh ), which in turn is parasitized by Alloxysta brassicae (Ashm. ),Asaphes vulgarisWalker ,A. suspensus (Nees ),Pachyneuron minutissimum (Förster ) and Dendrocerus carpenterii (Curtis ). The aphid population in the field was started by immigrant alates which were found flying too early to be synchronized with the sprouts plants. Similarly D. rapae was not synchronized with the aphids although many individuals could have been carried into a plot through parasitized immigrant alates, of which less than 30% were found parasitized. Because of high hyperparasitism (especially by A. brassicae)D. rapae was not able to maintain a high rate of parasitism to curb the aphid population growth. The maximum percentage mummies being 27.8%, while the maximum, percentage parasitism being 56.6% recorded only during the early 1974 season (mean=12.9%). The decline of aphid population from September onwards was largely due to the cold weather, Syrphid predation and occasionally fungal attack. The high rate of hyperparasitism by A. brassicae is attributed to its better synchronization with D. rapae. The mean percentage of parasite that emerged from mummies collected during 1973–74 were D. rapae 31.3%,A. brassicae 64.3%,A. vulgaris and A. suspensus 4.3%,D. carpenterii 0.2% and P. minutissimum 0.1%.